gnathostome was already a complex organism with a well- has shown some, or even marked, change, and these changes 



defined and many-parted brain; three pairs of main sensory appear to be largely adaptive specializations peculiar to the 



structures — nose, eye, ear; at least ten pairs of cranial nerves, group possessing them. As a possible exception, there has 



plus a pineal structure and nerve; complex trigeminal, been a trend toward increasing size and complexity of the 



facial, and vagus nerves; and numerous other details. In the brain in the tetrapods which suggests a phylogenetic series, 



evolution of the vertebrates each part of the nervous system but the details of this evolution are yet to be worked out. 



GENERAL OBSERVATIONS • 419 



